1. Field of the Invention
The present invention relates to a wavelength selecting apparatus and a wavelength selecting method.
2. Description of the Related Art
A tunable filter is conventionally placed in a photonic network as an important optical element used to select a wavelength required to split signal light. Among such optical elements used to select a wavelength, an acousto-optic tunable filter (AOTF) has particularly attracted attention in recent years. The AOTF is a type of optical filter that has merits, such as enabling a reduction in size and cost based on integration, in addition to high-speed properties of a wavelength selecting operation and extensiveness of a tunable range. A wavelength selecting apparatus based on this AOTF can be already put to practical use. However, an AOTF subsystem that includes a laser output unit and utilizes this laser as a reference light source to improve accuracy of wavelength selection based on the AOTF has also been proposed.
FIG. 9 is a view of a conventional AOTF subsystem. The AOTF subsystem includes a function unit that judges whether selection of a wavelength by an AOTF is appropriately performed and accordingly executes control. An AOTF subsystem 10 in the drawing includes an AOTF 11 that selects a wavelength, a reference light source unit 12, a monitoring AOTF 13, a light receiver 14, and a controller 15. This subsystem also includes a temperature controller 16 that executes control according to the temperature dependence of an element serving as a light source.
The AOTF subsystem 10 is configured to have both a wavelength selecting function of the AOTF, i.e., only transmitting signal light having a specific wavelength based on a radio frequency (RF) control signal input to the AOTF 11 from the controller 15 and a function of using reference light from the reference light source unit 12 to record a corresponding relationships between the RF control signal and the selected wavelength and deriving linear RF control signal-selected light wavelength characteristics to improve a selection accuracy for a light signal. According to such a configuration, when multiplexed light signals (λa to λh) are input to the AOTF subsystem 10, a light signal (λd) having a selected wavelength alone can be selected and transmitted.
FIG. 10 is a view showing a configuration of the AOTF. An operation of selecting a wavelength in the AOTF 11 will now be explained with reference to FIG. 10. The AOTF 11 and the monitoring AOTF 13 have the same element structure, however, they input signals to different ports depending on application, and hence they are individually depicted. The AOTF 11 includes a polarizing beam splitter (PBS) 20, a comb-like electrode 21, and a surface acoustic wave (SAW) waveguide 22 in a circuit manufactured on a lithium niobate (LiNbO3) substrate.
When an RF control signal is applied to the comb-like electrode 21, a SAW is excited. In the linear SAW waveguide 22 through which the SAW is transmitted, a polarized wave having a particular wavelength rotates due to an acousto-optic effect. Here, when the PBS 20 integrated at an intersection of the circuit operates without being dependent on the polarized light, input light (λa to λh) can be divided into selected light (λd) and non-selected lights (λa, λb, λc, λe, λf, λg, and λh) and led to different ports, respectively. A wavelength of the selected light is dependent on a frequency of the RF control signal applied to the comb-like electrode 21, and multiple selected lights can be obtained when multiple RF control signals are input. Selection of a wavelength by the AOTF 11 can be carried out by using the above-explained principle (refer to “Photonic Network” by Terumi Chikama, [online], July, 1999, Magazine FUJITSU, [retrieved Nov. 19, 2004, Internet <URL: http://magazine.fujitsu.com/index2.html>]).
As the reference light source unit 12, a unit configured by a light-emitting diode (LED) having a wide emission spectrum is disclosed in Japanese Patent Application Laid-open No. H3-233425, Japanese Patent Application Laid-open No. H6-120605, and Japanese Patent Application Laid-open No. H10-262031.
However, as a light source element of the reference light source unit 12 in the conventional AOTF subsystem 10, a distributed feedback-laser diode (DFB-LD) is used for its characteristic of outputting only one wavelength per element. This DFB-LD is advantageous in that a peak of an output waveform is prominent in the light receiver 14, and detection of the peak by the controller 15 is easy. On the contrary, the DFB-LD has problems, such as reduced reliability of the light source due to a temperature dependence or deterioration with time and a high cost. There is demand to configure the reference light source unit 12 using an element other than the DFB-LD. Furthermore, the peak of an output waveform cannot be detected when the LED alone is used as the reference light source unit 12 and the LED cannot be applied to the AOTF subsystem as it is.